I'm not familiar with ISS but the PADI 'qualification' is really rock bottom for bare minimum you need to start diving. After your four open water dives you're not really in any shape to go diving without an experienced buddy/professional to look after you.
Personally speaking, I started to get comfortable in the water around dive 20, confident around dive 50 and more confident around dive 200. From what I'm observed in a decade diving in Asia is that my experience is not atypical. I've only ever seen one diver halfway passable in the water after completing basic OW training.